Not very pretty watercolour? (2,3,8)

I believe the answer is:

no oil painting

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'not very pretty' is the definition.
('no oil painting' can mean an unattractive person)

I don't understand how the remainder of the clue works.

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(Other definitions for no oil painting that I've seen before include "that's inferior on the face of it" , "Not much to look at! - with 8d" , "Somewhat unattractive" , "Water colour" , "Portrait in water colours" .)

I've seen this clue in The Times.

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